This invention relates to lightning surge arresters, and more particularly to improvements in the performance, uniformity, cost and strength of such arresters.
Lightning arresters are protective devices used to limit surge voltages in equipment caused by lightning or other disturbances in the equipment circuit. Lightning arresters generally function to bypass or discharge surge current and discharge surge voltages within a fraction of a cycle and prevent damage to the protected equipment.
In very high voltage power transmission lines such as power transmission lines handling 500 KV or more, and long transmission lines or long cable lines having large line capacitance, the energy developed by switching surges is so large that arresters having a large discharge current rating are required. When arresters employing a series gap are used in such transmission lines, i.e., arrestors using non-linear resistors which are isolated from line voltage by spark or arcing gaps in series with the resistors, a series arcing gap having a large discharge current rating is required. Arresters using series arcing gaps having large discharge current ratings have many disadvantages in design, manufacturing, and maintenance.
The recent development of metal oxide, e.g., zinc oxide, resistor elements has resulted in significantly higher nonlinearity and has made possible the elimination of the series gap. In those arresters which do not employ a series gap, a stack of disk-like resistor elements constructed of zinc oxide are used. The diameter of the resistor elements can be increased to correspondingly increase the discharge current rating of the resistor stack, and accordingly the discharge rating of the arrester.
However, large diameter resistor elements have several disadvantages. For example, it is difficult to avoid internal stresses in the large resistor elements during manufacture and it has been found that large diameter resistor elements are subject to cracks. Hence, is it desirable to use smaller diameter resistor elements.
Non-linear resistor stacks in these arrestors can be connected in parallel to further increase the discharge rating of the arrester. In such arresters, it is desirable that the current flowing in the parallel resistor stacks be equal or nearly so for the arrestor to perform at its indicated rating. However, voltage-current characteristics of individual resistor elements are often unequal so that the resulting resistor stacks having the same number of resistor elements tend to have unequal voltage-current characteristics, particularly when a small number of elements are employed.
In many prior art devices, the inequality in voltage-current characteristics is so large as to require that the number of resistor elements forming the individual stacks be varied, i.e., resistor elements must be added to some stacks, to make the voltage-current characteristics of the parallel stacks more nearly equal. Then, means is required to compensate for the difference in length of the parallel resistor stacks. See, for example, Japanese patent disclosure (Kuokaikouhou) No. 27845/74. In general, the prior art does not provide a suitable solution to the problem of unequal voltage-current characteristics in parallel resistor stacks.